The simile to which you refer, "words as hard as cannon balls," was written by Ralph Waldo Emerson in his famous essay entitled "Self-Reliance." The theme of this writing, as well as most of Emerson's other works, is that we are all on a journey of growth and that we must learn to trust our own judgments and instincts as we make that journey.
Of course, one of the inevitable things that happens on every journey is change, and change often prompts contradiction. While some people are bothered by contradictions in a world view or belief system, Emerson contends that because change is inevitable, so is contradiction.
Others would argue that a belief system which holds steadfastly to certain basic principles will not cause such contradictions because every decision and thought is connected to that set of guiding principles. Emerson, though, is one who celebrates the individual and each person's right to individuality. For example, he said:
- Make your own Bible. Select and collect all the words and sentences that in all your readings have been to you like the blast of a trumpet.
- It is easy to live for others, everybody does. I call on you to live for yourself.
- Trust thyself: every heart vibrates to that iron string.
His consistent message is to follow your own heart and do what it leads you to do, no matter who else agrees with you or what the consequences might be.
In "Self-Reliance," he uses a simile which says essentially the same thing. A simile, as you know, is a comparison between two things which are mostly unlike one another but do have at least one point of comparison. In this case, Emerson makes an unlikely comparison between words and cannon balls shot from a cannon. The entire quote reads this way:
Speak what you think today in words as hard as cannon balls and tomorrow speak what tomorrow thinks in hard words again, though it contradict everything you said today.
The comparison, then, is between words and cannon balls.
Cannon balls have one purpose--they are weapons used to fight an enemy. They are hard and, when they are propelled forcefully through a cannon, they are a powerful weapon of destruction. Words, too, can be used as weapons, and we have all been attacked by them in one for or another. In this simile, however, it is not the aspect of weapons which Emerson seems to be highlighting.
Instead he seems to be telling us to speak our words powerfully (hard) like a cannon ball is used powerfully. Let's put it another way: if you have ever been hit by a cannon ball, you know it because of its power and force; the same can be true of words. Emerson is suggesting that we avoid timidity and fear, speaking our views (our truth) today in the strongest possible terms; we should do the same tomorrow, even if our words reflect something different than we spoke today (a new truth).
Do not be shy, he says, about loudly proclaiming today's truth just because you know tomorrow's truth might be a direct contradiction. Speak both truths, all truths, loudly and forcefully, like a cannon ball. For many, this would seem to be a contradiction or inconsistency, but his philosophy is this:
A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines. With consistency a great soul has simply nothing to do.
Speak your truth like a cannon ball every day, he says, and any contradiction simply shows that you are growing and changing on your life's journey.